Do you have a quote for that, because I don't recall anything about it having anything to do with the Cardassian peace treaty and Memory Alpha only seems to have this to offer:
"The Federation refused to become involved in ending the Occupation, regarding it as an internal political matter and therefore covered by the
Prime Directive, despite the fact that
they themselves had been at war with the Cardassians for much of the latter half of the Occupation. (
TNG: "
Ensign Ro")" - Funny how the Federation's normally ok with ENDING prime directive violations, yet they labeled any interference in the occupation as a PD in and of itself...
"After fifty years of occupation, the Cardassians withdrew from Bajor in 2369. The exact cause of the withdrawal is largely a matter of opinion; while the Bajorans attributed it to the efforts of the Bajoran Resistance, the Cardassians regarded it as an entirely political decision. In the days leading up to the withdraw, the Cardassians diverted warships away from Bajor in an attempt to annex
Minos Korva, a disputed planet near the Cardassian-Federation border. (
TNG: "
Chain of Command, Part I") After a round of tense diplomatic negotiations, Captain
Edward Jellico dealt the Cardassians a humbling defeat at the
McAllister C-5 Nebula. Under Jellico's terms of surrender, all of the ships were forced to eject their primary phaser coil, effectively rendering their ships defenceless. (
TNG: "
Chain of Command, Part II") Regardless of the cause, all sides acknowledge that civilian leaders such as those on the
Detapa Council made the decision, which was opposed by the Cardassian military. Gul Dukat in particular remained intent on reconquering Bajor for over five years. (
DS9: "
Duet", "
Cardassians", "
Call to Arms")." - So, if this is accurate, then, at best, a divided Cardassian govt. made a political decision to withdraw from Bajor as a part of trying to step back from wartime footing after a crushing defeat. Which has nothing to do with treaties forcing them to withdraw because the Bajoran occupation was ever considered illegal by anyone (other than the Bajorans).
As already pointed out by others, you're overinflating the number of planets involved and massively overstating the 'evidence' that they really haven't interefered in any of those planets.
It's also worth pointing out that your argument is based on another ridiculous assumption: that the planets we've seen on the show are necessarily the planets that the Klingons and Romulans would be most interested in interfering with. Space is huge and the Federation is not everywhere (certainly the Enterprise isn't, despite the whole only ship in the sector thing). They could've devastated thousands of pre-warp civilizations without Enterprise or the Federation ever hearing a thing about it.
In reference ot the Romulans in particular, we actually know of two huge gaps of time in which they had almost no activity anywhere near the Federation and during which the Federation has no idea what the Romulan Empire was doing. But we as the audience know that the Romulan Commander was a veteran of hundreds of campaigns before his ship was ordered to test the Federation, so they certainly weren't sitting at home knitting. I believe the second Romulan absence was also explained with a statement along the lines of 'our attention has been elsewhere', which also suggests some kind of significant activity far away from the Federation.
